I didn't think it mattered whether Costco took AMEX or not because Costco isn't the one processing the payment with AMEX. All Costco will do is call and give the card number to
DCL, then DCL processes the payment. If there needs to be a refund, it works the same way. DCL will process a refund to AMEX. You aren't paying Costco first and then Costco paying DCL. The Costco TA is just relaying the payment information. Just like Costco doesn't accept Disney gift cards for payment at their stores, but you can use Disney gift cards to pay for DCL cruises when using Costco as your TA.